1. Field of the Invention
This invention relates to an energy recovering technique, and more particularly to energy recovering apparatus and method that are capable of reducing a power consumed during sustaining discharge.
2. Description of the Related Art
A plasma display panel(PDP) is a device for displaying a picture by utilizing a gas discharge. The PDP provides a large-scale screen as well as an improved image quality owing to the recent development. The PDP is largely classified into a direct current(DC) driving system performing an opposite discharge and an alternating current(AC) driving system performing a surface discharge in accordance with its driving strategy. The AC driving system has been highlighted because it has a lower power consumption and a longer life time than the DC driving system. The PDP of AC driving system is intervened with a dielectric to apply an AC voltage and performs a discharge every its half period. The AC driving system is classified into a sub-frame system and a sub-field system. When 256 gray scales are expressed, the sub-field system makes a time division of one frame into 8 sub-fields. Each sub-field is time-divided into a reset interval for initializing the entire screen, an address interval for writing a data while scanning the entire screen in a line-sequence manner and a sustaining interval for sustaining a luminous state of cells into which the data is written. A time is assigned such that the reset interval and the address interval of each sub-field are same at each sub-field while the sustaining interval increases at a ratio of 2.sup.n (n=0, 1, 2, 3, 4, 5, 6 or 7) depending on a relative ration of the brightness. The gray scales proportional to the corresponding sustaining interval is implemented at each sub-field and the gray scales implemented at each sub-field are combined, thereby expressing 256 gray scales from one frame.
The sub-field system has a problem in that a power consumption is large at the time of charging or discharging the PDP in the sustaining interval. In the PDP of AC driving system, its driving circuitry includes an energy recovering circuit for recovering a voltage discharged from the panel again to charge the panel.
Referring to FIG. 1, there is an energy recovering apparatus that includes a scanning/sustaining electrode unit driving cell 10, hereinafter referred to as "Y electrode unit driving cell", connected to a Y electrode 1, and a common electrode unit driving cell 20, hereinafter referred to as "Z electrode unit driving cell", connected to a Z electrode 2. The Y electrode 1 and the Z electrode 2 are connected to a panel capacitor Cp. The panel capacitor Cp equivalently represents an electrostatic capacity formed between the Y electrode 1 and the Z electrode 2. The Y electrode 1 and the Z electrode 2 are discharged by a sustaining pulse applied to the Y electrode unit driving cell 10 and the Z electrode unit driving cell 20. The Y electrode unit driving cell 10 includes an external capacitor Cex1 connected to a ground terminal GND, first and third switches S1 and S3 connected, in parallel, to the external capacitor Cex1, second and fourth switches S2 and S4 connected, in series, between a sustaining voltage supply Vs and the ground terminal GND, and an inductor L1 connected between a first node n1 and a second node n2. The Z electrode unit driving cell 20 has the same configuration as the Y electrode unit driving cell 10 and is connected to the panel capacitor Cp in such a manner to be symmetrical to the Y electrode unit driving cell 10. Specifically, the Z electrode unit driving cell 20 includes an external capacitor Cex2 connected to a ground terminal GND, fifth and seventh switches S5 and S7 connected, in parallel, to the external capacitor Cex2, sixth and eighth switches S6 and S8 connected, in series, between a sustaining voltage supply Vs and the ground terminal GND, and an inductor L2 connected between a third node n3 and a fourth node n4. Diodes D1, D2, D3 and D4 connected to the first node n1 and the fourth node n4 are responsible for limiting a backward current.
The operation of the energy recovering apparatus will be explained on a basis of the Y electrode unit driving cell 10 with reference to FIG. 2. When the panel capacitor Cp is charged and discharged several times by the sustaining pulse, a voltage is charged in the external capacitors Cex1 and Cex2. In a t1 interval, the first switch S1 is closed. Then, a voltage charged in the external capacitor Cex1 is applied, via the first switch S1 and the inductor L1, to the inductor L1. Since the inductor L1 constructs a serial LC resonance circuit along with the panel capacitor Cp, the panel capacitor Cp begins to be discharged by a LC resonance waveform. At this time, the eighth switch S8 of the Z electrode unit driving cell 20 has been closed. In a t2 interval, the second switch S2 is closed at a resonant point of the LC resonance waveform. Then, since the sustaining voltage Vs is applied to the panel capacitor Cp, the panel capacitor Cp maintains a sustaining voltage level. A discharge is caused between the Y electrode 1 and the Z electrode 2 in a time interval when the panel capacitor Cp maintains a sustaining voltage level. In a t3 interval, the second switch S2 is opened and the third switch S3 is closed and thus the panel capacitor Cp begins to be discharged. At this time, a voltage charged in the panel capacitor Cp is applied, via the inductor L1 and the third switch S3, to the external capacitor Cex1 to charge the external capacitor Cex1. Next, the fourth switch S4 is closed. Then, a voltage of the panel capacitor Cp drops into a ground voltage. The Z electrode unit driving cell 20 charges and discharges a panel capacitor Cp alternately with the Y electrode unit driving cell 10.
As a result, the energy recovering apparatus recovers a voltage discharged from the panel capacitor Cp by utilizing the external capacitors Cex1 and Cex2 and applies it to the panel capacitor Cp, thereby reducing an inordinate power consumption during the sustaining discharge. Since an efficiency and the brightness of the PDP are basically influenced by a current, they has a limit as long as the panel capacitor Cp is charged by means of voltage sources such as external capacitors Cex1 and Cex2.
On the other hand, in the sub-frame driving system, an addressing interval of the entire screen is distributed partially every period of a sustaining pulse to continue the sustaining process without an interruption. In the sub-frame system, when it is intended to express the 256 gray scales, the entire screen is divided into 8 time regions T, T/2, T/4, T/8, T/16, T/32, T/64 and T/128 in the horizontal direction and a discharge weighting value is assigned to each time region in similarity to the sub-field system. Accordingly, at an optional time, 8 screen blocks with a different brightness level, that is, in a different sub-field state exist in the entire screen. 8 scanning lines selected in one sustaining pulse period repeats a process moved downward by one scanning line each time the sustaining pulse period is changed. In such a sub-frame driving system, the sustaining pulse includes a level of more than three steps such that it has a reference level of a writing pulse and a reference level of an erasing pulse every period. Since the energy recovering apparatus as shown in FIG. 1 is connected in cascade as many as the number of step so as to generate a sustaining pulse having a level of more than three steps, the configuration of the driving circuitry becomes complicated.